Method for the assembly of small electrical machines

ABSTRACT

The method and the device are designed for assembling a stator (14), formed by a stack (16) of core plates and its windings (18), on to a respective head support (12) of a small, rotary electric machine, in a position concentric with a rotary shaft (20) mounted in the support. The method is of the type in which the periphery of one end face of the stack (16) of laminations rests on coplanar peripheral surfaces (22) of the support (12) which define a reference plane (P), a centering device (34) is interposed between the shaft (20) and the stack (16), and the stack (16) and the support (12) are held together by means of peripheral bolts (24) which are inserted in regions located in correspondence with the coplanar surfaces (22) of the support (12), are perpendicular to these surfaces (22), and are screwed into the support (12). Before the bolts (24) are tightened, the stack (16) is clamped between, at one end, the coplanar surfaces (22) and, at the other end, thrust members (52) applied to the other end face of the stack (16) in regions also corresponding with the regions in which the bolts (24) are inserted, with a clamping force normal to the reference plane (P) and at least substantially equal to that predetermined for tightening the bolts (24), the bolts (24) are then tightened with the predetermined tightening force, and the clamping force applied by the thrust members (52) is released.

The present invention relates to the manufacture of rotary electricalmachines and has been developed for the manufacture of hermeticmotor-compressor units for refrigerant fluids.

It relates, in the first place, to a method for assembling a stator,formed by a stack of core laminations and windings therefor on arespective head support of a small rotary electrical machine in aposition concentric with a rotary shaft mounted in the support. Theperiphery of one end face of the stack of laminations rests onperipheral coplanar surfaces or the support which define a referenceplane. A centering device is interposed between the shaft and the stackand the stack and support are held together by means of peripheral boltsinserted in regions corresponding to the coplanar surfaces of thesupport, perpendicular to the surfaces, and are screwed into thesupport.

A method of this type is known from the document IT-A-996.643. In thisdocument, a centering device is described, inter alia, which includestwo concentric, "tulip"-type collets, of which the inner one is designedto contract around the shaft and the outer one is designed to expandagainst the surface of the bore in the stack of laminations.

This collet ensures very accurate centering of all the laminations inthe stack relative to the shaft before the stack is clamped by means ofthe bolts. When the bolts are tightened, however, the clamping torque istransmitted, by friction, from the heads of the bolts to theimmediately-adjacent lamination, causing the first lamination to beforced radially against the outer collet; this force is transmitted byfriction to successive laminations so that the laminations are forcedalong a generatrix against the outer collet and, when this is released,the accuracy of centering obtained thereby is cancelled out. Thisinaccuracy means, on the one hand, that larger air gaps have to beprovided in the design stage and this reduces the efficiency of theelectrical machine. What is worse in the case of a non-synchronousmotor, is that the mutual displacement of the laminations caused bytightening of the bolts causes irregularities in the air gaps resultingin the motor being unable to reach its starting torque under load. Thisproblem is particularly serious in the case of small hermetic motorcompressors for refrigerators, particularly considering the fact thattesting of the motor is carried out only on the finished compressor,that is when the compressor motor is enclosed in its hermetic casing. Inthis case, rejection of the compressor due to defects in its motorinvolves a cost corresponding to that of the finished product.

Currently, in order to avoid displacement of the laminations duringtightening of the bolts, in small electric motors, before theconstruction of the windings, the stator stack is impregnated with athermosetting resin whose hardening results in a monolithic stack inwhich the laminations can no longer be displaced by the tighteningtorque. Methods of this type are known, inter alia, from the documentsU.S. Pat. No. 3,356,360 and U.S. Pat. No. 3,573,129.

Impregnation is a fairly expensive operation both in capital expenditureand in time: from the latter point of view, it constitutes a lengthyadditional step in the manufacture of an electrical machine.

According to another method, known from the document FR-A-1,452,093, astator stack of laminations is fitted around an expandable collet of the"tulip"-type. After the collet has been expanded in the bore of thestack, the latter is submitted to axial compression. While the stack iscompressed the laminations are welded together on the periphery of thestack to obtain a monolithic stack.

The document FR-A-2,093,227 discloses a method similar to that ofFR-A-1,452,093. According to FR-A-2,093,227 use is made of anon-expandable mandrel and the laminations of the stack under axialcompression, with the mandrel fitted in the bore of the stack, arewelded along welding lines adjacent to the bolt holes.

The methods according to FR-A-1,452,093 and FR-A-2,093,227 have thedrawback of requiring welding operations which constitute a lengthyadditional step in the manufacture of an electrical machine.

The main object of the present invention is to provide a method whichenables a small electrical machine to be produced with a very small,regular air gap between the stator and the rotor without the need toimpregnate or weld the core stack.

According to the present invention, this object is achieved by means ofa method wherein before tightening the bolts, the stack is clampedbetween the coplanar surfaces on one side and thrust members on theother side, said thrust members being applied to the other end face ofthe stack in regions also corresponding with the regions of insertion ofthe bolts, with clamping forces normal to the reference plane and atleast substantially equal to those predetermined for the tightening ofthe bolts. The bolts are then tightened with the predeterminedtightening force and the clamping force applied by the force members isthen released. Thanks to this solution the following takes place:

before tightening of the bolts, the stack, which has already beencentered around the shaft with the maximum possible precision by acentering device such as the double collet of the document IT-A-996,643,is put into substantially the same final condition of compression asthat in which the bolts will hold the stack after they have beentightened;

since the bolts are tightened on to the compressed stack, the mutualfriction between laminations, due to compression by the thrust members,prevents them from sliding relative to each other.

From the documents U.S. Pat. No. 3,356,360 and U.S. Pat. No. 3,573,129,compression of an unwound stack, by means of a tool, with a view toimpregnating it with a thermosetting resin is known but this is toachieve a different object from that of the present invention. Indeed,according to these documents, the object of the precompression is toreduce the quantity of resin necessary to impregnate the pack. Afterhardening of the resin, the precompression caused by the tool isreleased. In other words, this precompression is not used to put thestack of an already-wound stator into the condition in which it will beafter tightening of the bolts.

The document U.S. Pat. No. 4,290,291 discloses an apparatus in which astack of laminations of an already-wound stator is submitted to axialcompressive forces between pads that simulate mounting pads in acompressor. The compressive forces simulate and closely approximate thecompressive forces that ultimately will be applied to the bolt holeregions under the heads of mounting bolts in a compressor. Variouseccentricities and dimensional variations between the bore and boltholes are transferred to the bore. While the stack is held with thepreselected compressive force, the bore is shaped with a rollerburnisher so that it will be concentric relative to the bolt holes.

This method does not solve the problem of the invention. After theburnishing step, which represents an additional cost, the compressiveforces are released and the stator is no longer in the condition inwhich it will be after tightening of the bolts. Thus, also in this casethe tightening of the bolts gives rise to the same drawbacks mentionedin connection with the prior art of IT-A-996,643, that is the loss ofaccurate centering obtained by the burnishing.

The invention also relates to a device for device for carrying out themethod and to a small electrical machine and a unit including such amachine, produced by the method.

By the term unit is meant particularly, but not exclusively, amotor-compressor for refrigerant fluids in which the support for thestator is constituted by the body or crankcase of the refrigerant-fluidcompressor.

The invention will be understood more clearly from a reading of thedetailed description which follows, with reference to the attacheddrawings which show, by way of non-limiting example, the device and themethod according to the invention in their application to the assemblyof a motor-compressor unit for refrigerant fluids.

In the drawings:

FIG. 1 is a perspective view of a motor-compressor unit arranged for thestator stack of its electric motor to be centered and clamped in adevice according to the invention, in which the principal parts of thedevice designed to cooperate with the unit to carry out the methodaccording to the invention are also shown in the positions in which theyare situated before the method is carried out.

FIG. 2 is a partially-sectioned elevational view showingdiagrammatically the motor-compressor unit and the parts of the devicein the same positions as in FIG. 1.

FIG. 3 is a view similar to that of FIG. 2 showing the motor-compressorunit and the parts of the device in a position corresponding to apreparatory step of the method.

FIG. 4 is a view similar to that of FIGS. 2 and 3 showing themotor-compressor unit and the parts of the device in a first step of themethod,

FIG. 5 is a plan view seen as indicated by the arrow V of FIG. 4 showingthe motor-compressor unit and certain parts of the device in the sameconditions as in FIG. 4, on an enlarged scale,

FIGS. 6 and 7 are elevational views similar to the above which show themotor-compressor unit and the parts of the device in two subsequentsteps of the method, and

FIG. 8 is a section taken in the plane indicated by the line VIII--VIIIof FIG. 7 on an enlarged scale, showing the unit and parts of the devicein the same conditions as in FIG. 7.

Particular reference will be made to FIG. 1 to describe themotor-compressor unit, generally indicated 10, which is submitted to themethod according to the invention and the principal members of thedevice for carrying out the method. The various parts of the unit and ofthe device also appear in the other drawings which may be referred to,and in which they are indicated by the same reference numerals.

The motor-compressor unit 10 includes a body or crankcase 12 consistingof an iron or light alloy casting. The body 12 acts as a head supportfor the stator, generally indicated 14, of an electric motor. The stator14 comprises a stack of core laminations 16 with its windings 18.

The body 12 also supports a rotary shaft 20 which constitutes both theshaft of the electric motor and the crankshaft of the compressor, ofwhich the casing 12 also forms a part.

One end face of the stack 16 rests on ground, coplanar peripheralsurfaces 22 of the body 12. These surfaces 22, for the reason which willbe explained below, project relative to the periphery of the stack 16.

Bolts 24 extend through the stack 16 in correspondence with the coplanarsurfaces 22. The axes of the bolts 24 are normal to the plane defined bythe surfaces 22.

The bolts 24 are screwed into corresponding threaded holes in the body12. Their function is to clamp the laminations of the stack 16 togetherand to fix the stack 16 firmly to the body 12. A further function of thebolts 24 is to fix respective annular caps 26 to the stack. The stacks26 are designed to receive helical springs for suspension of the unit 10(upside-down relative to the position shown in FIG. 1) in the hermeticcasing of a compressor.

The main components of the device according to the invention will now bedescribed in its use with a motor-compressor unit such as that indicated10.

The device includes a lower assembly consisting essentially of means forsupporting the body 12 in the arrangement illustrated in FIGS. 1 to 4, 6and 7, in which the coplanar surfaces 22 are substantially horizontaland facing upwards. In the embodiment shown, the support means comprisethree columns 28 which are movable vertically upwards and downwards, aswill be better explained below. At its upper end, each column 28 carriesa respective shaped support 30 on which the body 12 rests.

At the lower end of each column 28 there is a respective thrust member(not shown) for driving the vertical upward and downward movement of therespective column and for exerting an upward reaction force on the body12 through the respective column 28 and its support 30. More will besaid about this force below. For now, it will suffice to say that thethrust members are independent of each other both with regard to theirtravel in operation and to the generation of the force exerted.

The thrust members preferably consist of linear fluid actuators, butwedges movable transversely to the direction of movement of the columns28 and their supports 30 could be used instead.

The device according to the invention also includes an upper assemblythe principal parts of which will now be described.

The upper assembly comprises a central column 32, movable verticallyupwards and downwards, which supports beneath it a double collet 34 ofthe "tulip"-type, described and shown in detail in the documentIT-A-996,643 to which the reader is referred.

Some details of the double collet 34 are shown in FIG. 6 to whichreference will be made briefly.

The double collet comprises an inner collet 36 and an outer collet 36.The inner collet 36 is radially contractible and arranged to grip andcentre the shaft 20. The outer collet 38, on the other hand, is radiallyexpansible and designed to engage the surface of the bore in the stack16 and to centre the stack relative to the shaft 20. Contraction of theinner collet 36 is caused by means of a conical bush 40 and expansion ofthe outer collet 38 is caused by means of a conical bush 42.

The two bushes 40 and 42 are carried by the lower ends of respectivetubular, concentric shafts which are slidable within the column 32 andare driven by respective linear fluid actuators, not shown. Anotheractuator, not shown, causes the column 43 and the double collet 34 todescend and rise.

The upper assembly also includes pneumatic screwdrivers 44 for screwingthe respective bolts 24. Each screwdriver essentially comprises a rotaryshaft 46, movable vertically along its axis and carrying a screwing heador socket wrench 48 at its lower end. The upper ends of the shafts 44are associated with respective pneumatic motors or the like. Thescrewing heads 48 are arranged to engage the hexagonal heads of thebolts 24 within the caps 26.

The upper assembly also includes a pressure plate 50 which is movablevertically upwards and downwards independently of the shafts 46 and hasholes for the passage of these shafts. Sleeve-shaped thrust members 53are associated with the plate 50. The sleeves 52 are slidable in theplate 50 in the direction of its movement and react against the plateitself, with the interposition of disc compression springs 53,preferably of the Bauer type.

The sleeves 52 have respective lower radial faces 54 which, as will bebetter explained below, are arranged to engage the upper lamination ofthe stack 16 in the region immediately surrounding the caps 26. In theembodiment shown, the sleeves have a lateral rotch 55 so as not tointerfere with the head of the winding 18 during the compression of thestack 16 which will be described below.

A fluid actuator is associated with the pressure plate 50 and isarranged to cause both vertical travel of the sleeves 52, independentlyof the travel of the screwdrivers 44, and to exert a clamping force, ofwhich more will be said below, on the stack 16 by means of the springs53 and the radial faces 54 of the sleeves 52.

The device according to the invention also includes a pair of oppositelateral slides 56 which are movable horizontally towards the unit 10carried by the support means 28, and in the opposite direction. Each ofthe slides 56 carries a pair of fingers 58 in the form of rockers, eacharranged to pivot slightly in a vertical plane. Each finger 58 has adownwardly-facing shoulder surface 60 against which one of the coplanarsurfaces 22 of the body 12 is intended to abut for the purpose whichwill be explained below. For now, it will suffice to say that eachfinger 58 can pivot between a rest position, in which it is slightlylowered as in FIGS. 2 and 3, and a slightly raised position, as in FIGS.4, 6 and 7. This raised position is determined by adjustment screws 62in such a way that the shoulder surfaces 58 define a reference plane Pwhich is strictly perpendicular to the vertical axes of the componentsof the upper unit of the device and, in particular, to the vertical axesof the sleeves 52.

Reference will now be made to FIG. 2 and to subsequent Figures todescribe the operation of the device and the method carried out thereby.

The device preferably consists of a work station incorporated in aassembly line in which units such as 10 are transported in a directionnormal to the plane of the drawing of FIG. 2.

When a unit 10 arrives at the station, it is placed, in a manner notshown, on the supports which, together with their columns 28, are in alowered position.

In this preliminary step of FIG. 2, the whole of the upper assembly israised and the slides 56 are moved apart and away from the unit 10.

It should be mentioned that, in the unit 10 which arrives at thestation, the bolts 24 are screwed into the body 12 but not tightened sothat the laminations of the stack 16 are loose.

In a subsequent step of the method, the slides 56 are moved closertogether, in the direction of the arrows A of FIG. 3, so that theshoulder surfaces 50 of the fingers 58 overlap the coplanar surfaces 22of the frame 12.

In a further subsequent step illustrated in FIG. 4, the three columns 28are moved upwardly in the direction of the arrow B until the coplanarsurfaces 22 abut the shoulder surfaces 60. In this way, the fingers 58effect their small angular upward movement until the point at which theshoulder surfaces 60, and hence the coplanar surfaces, are brought withmaximum precision into the plane P which, from this point onwards,constitutes a reference plane for the whole of the rest of theoperation.

The situation of FIG. 4 is clarified by the plan view of FIG. 5.

In the next step of the method illustrated in FIG. 6, the whole of thedouble collet 34 is moved downwardly in the direction of the arrow Cuntil it is interposed between the shaft 29 and the surface of the borein stack 16. The inner collet 36 is next contracted around the shaft 20and then the outer collet 38 is expanded against the inner edges of thelaminations which define the surface of the bore in the stack 16. Inthis operation, all the laminations of the stack 16 are centered asexactly as possible about the shaft 20 by virtue of the fact that thelaminations, being loose, can slide on each other.

In the next step, illustrated in FIG. 7, the pressure plate 50 is moveddownwardly in the direction of the arrow D until the radial faces 54 ofthe sleeves 52 abut the upper lamination of the stack 16 around the caps26.

The faces 54, because of the actuator associates with the pressure plate50, exert a clamping force normal to the reference plane P between thesesurfaces and the coplanar surfaces 22 of the body 12. This clampingforce is equal to or at least substantially equal to that predeterminedfor subsequent tightening of the bolts 24. By virtue of the springs 53,both the travel in operation and the clamping force exerted by eachsleeve 52 are independent of those of the other sleeves 52.

The reaction force exerted by the thrust members associated with thelower columns 28 is such as to ensure that the coplanar surfaces 22remain firmly applied against the shoulder faces 60 of the fingers 58,that is that they remain in the reference plane P.

The plan view of FIG. 8 shows, inter alia, circular sector-shaped dottedareas 54a where the radial faces 54 exert pressure on the upperlamination of the stack 16.

As will be seen, there are two areas 54a in the neighbourhood of eachbolt 24, located substantially symmetrically about the axis of the bolt24 so that the resultant of the forces exerted thereby is substantiallyon the axis of the bolt itself.

Whilst the stack 16 is clamped by the thrust members constituted by thesleeves 52, the screwdrivers 44 are lowered (arrow E) and their screwingheads 48 engage the heads of the bolts 24.

Finally, the shafts 46 of the screwdrivers 44 are rotated (arrow F),fully tightening the bolts 12 with a torque such as to exert thepredetermined clamping force on the pack by means of the boltsthemselves.

The method of assembling the stator 14 on its end support constituted bythe body 12 of the compressor is thus completed and the variouscomponents of the device are returned to the positions of FIG. 2. Thebolted unit 10 is then moved away on the conveyor line and sent on to astation for fitting the rotor into the stator cavity.

I claim:
 1. A method for assembling a stator (14) comprised of a stackof core laminations (16) and windings (18) therefor on to a respectivehead support (12) of a small rotary electrical machine in a positionconcentric with a rotary shaft (20) mounted in the support (12), inwhich the periphery of one end face of the stack (16) of laminationsrests on peripheral coplanar surfaces (22) of the support (12) whichdefine a reference plane (P), a centering device (34) is interposedbetween the shaft (20) and the stack (16), and the stack and the supportare held together by means of peripheral bolts (24) inserted in regionscorresponding with the coplanar surfaces (22) of the support (12),perpendicular to these surfaces, and are screwed into the support,characterised in that before the bolts (24) are tightened, the stack(16) is clamped between the coplanar surfaces (22) on one side andthrust members (52) on the other side, said thrust members (52) beingapplied to the other end face of the stack (16) in regions alsocorresponding with the regions of insertion of the bolts (24), withclamping forces normal to the reference plane (P) and at leastsubstantially equal to those predetermined tightening forces to beapplied by the bolts on the stack, the bolts (24) are then tightened toapply said predetermined tightening forces on the stack and the clampingforces applied by the thrust members (52) are then released.
 2. A methodaccording to claim 1, characterised in that before the stack (16) isclamped by means of the thrust members (52), the regions of the coplanarsurfaces (22) of the support (12) which project from the periphery ofthe stack (16) are applied against shoulder surfaces (60), which arealso coplanar, by irreversible forces reacting in the direction oppositeto the force which will be exerted by the thrust member (52), and inthat forces are exerted by the thrust members (52) normal to the planedefined by the shoulder surfaces (22).
 3. A method according to claim 2,characterised in that the reaction forces are exerted by means of thrustmembers which are independent of each other with respect both to theirtravel in operation and to the generation of the force exerted.
 4. Amethod according to claim 1 characterised in that thrust members (52)are used which are independent of each other with respect both to theirtravel in operation and to the generation of the clamping force exerted.5. A method according to claim 1 characterised in that pressures exertedby the thrust members (52) are distributed in correspondence with eachbolt (24) in two areas (54a) which are symmetrical about the axis of thebolt (24).